Various systems have been proposed in connection with measuring-while-drilling (MWD) and logging-while-drilling (LWD) that use the drill string as a medium for transmitting sonic signals to the surface that represent a downhole measurement. None of these proposals are believed to have achieved any significant commercial use in the industry, and many of them no actual use at all. The principle reason why such prior systems have not been successful is believed to be that the drill string acts like a mechanical filter which significantly attenuates the sonic vibrations such that little or no useful information ever reaches the surface. In attempts to solve this problem, much research has been done on systems that employ repeaters which receive, amplify and retransmit the sonic signals at various levels in the drill string with the objective of having useful information reach the surface. However, in addition to being very expensive, systems that use repeater stations are believed to have encountered various technical difficulties. For example, the system disclosed in Matthews U.S. Pat. No. 4,066,995 issued Jan. 3, 1978 employs noise isolator subs at various points in the drill string to filter out background noise signals, and these subs create mechanical damping which does not allow operation of resonating transducers. Nardi U.S. Pat. No. 4,283,780 issued Aug. 11, 1981, and Kent et al U.S. Pat. No. 4,302,826 issued Nov. 24, 1981 use a mass/spring resonant system excited by a piezoelectric source and directly coupled to the steel of the drill string. These devices require an electric resonating circuit for exciting the transducer, and must be fine-tuned with respect to both the electrical and mechanical systems. U.S. Pat. No. 3,103,643 issued Sep. 10, 1963 to Kalbfree requires a special drill pipe joint to be operable. Other patents such as Shawhan U.S. Pat. No. 3,930,220 (1975) suggest telemetering from downhole to the surface using the drill string and repeaters at various levels therein as discussed above, while other patents such as U.S. Pat. Nos. 3,697,940, 4,562,559 and 3,900,827 relate to similar systems. All such proposals have the disadvantage of requiring a complicated and expensive drill string.
Although transmission from bottomhole to the surface with a single sonic transmitter has not been found to be practical, applicant has found that telemetry via sonic vibrations transmitted through the steel members of the bottom hole assembly of the drill string over a relatively short communication link can be very useful, for example between a sensor sub that is positioned near the drill bit and an MWD tool that is positioned further uphole. This sensor sub is described in detail in commonly-assigned U.S. patent application Ser. No. 07/823,789, filed concurrently herewith and hereby incorporated herein by reference. The MWD tool operates to produce encoded pressure pulses in the mud stream inside the drill string which can be detected at the surface in a highly reliable manner. The metal members between the lower and upper ends of this link are typically drill collars having the same outer diameters. Attenuation of sonic vibrations is very low when using these members as a transmission medium. Indeed, it has been found that with the bit off bottom so that the background is relatively quiet, it is possible to transmit sonic vibrations and reliably detect them over a substantial distance, provided the diameters of the steel pipe members are substantially the same. Even during the drilling process, transmission over a distance of about 250 feet can be accomplished, limited primarily by the transmitting power of the system rather than attenuation of the signals or the high noise of the drilling process. The transmission properties of the drill collar steel are essentially independent of borehole conditions, and the transmitter and receiver should be operated at a frequency that is well above the frequency range of most of the noise generated by the drilling process. For example, the transmitter of the present invention operates at a resonant frequency that is above 10 KHz, and preferably as high as 25 KHz. A modulation system is employed such that a ceramic crystal transmitter produces bursts of sonic vibrations that are digitally encoded in terms of their repetition rates. The signals that are detected at the MWD tool arrive under conditions that provide a very favorable signal-to-noise ratio. It is within the scope of the present invention for such signals to be detected, amplified and then transmitted further uphole by telemetry other than mud pulse, for example by sonic repeater stations spaced axially along the drill string. New and improved sonic signal transmitter and receiver apparatus also are disclosed, as well as unique encoding and decoding systems.
A general object of the present invention is to provide a telemetering system by which measurements that are made near the bottom of a borehole are telemetered to the surface by means of modulated sonic vibrations created in the drill string members.
Another object of the present invention is to provide a system of the type described that operates at a predetermined frequency so as to be readily detectable over relatively high level background noise, for example that level of noise that is generated during the drilling process.
Another object of the present invention is to provide a sonic vibration transmitter that produces discrete bursts of vibrations which are digitally encoded in terms of repetitive rate to represent a downhole measurement.
Still another object of the present invention is to provide transmitter and receiver apparatus that are constructed and arranged to provide highly efficient coupling of sonic vibrations to and from a metal member of a drill string.